The present invention provides a practical design of a megavoltage x-ray detector with both high quantum efficiency (QE) and high resolution. The x-ray detector includes an optical-fiber taper (OFT) made from a large number of optical fibers, each of which is aligned with the incident x-rays from an x-ray source hitting a top surface of the optical fiber taper. The optical-fiber taper is a matrix of optical fibers with the core material made of, e.g., silica and coated with a cladding glass or polymer such that light created within the core of each optical fiber will be guided to the bottom ends of the fiber with the ends of the fibers at the bottom being optically coupled to and optical image read-out device. Each optical fiber in the optical fiber taper is fully aligned with the incident x-ray source so that x-rays entering the top of the fiber travel directly towards the bottom of the same fiber. This alignment (or focusing) of the optical-fiber taper towards the x-ray source can be achieved by an extra coating at the bottoms of the optical fibers so they have a larger diameter than the other top ends of the fibers.